If you click one of the "Subscribe to:" links along the bottom of a post, you will be notified by email when a new post and/or comment is posted associated with the subscription you chose. You will be notified once per day of any new posts that you've subscribed to. By default you are automatically subscribed to any post that you create or post a comment to.

To change your subscription and notification settings, go to your profile (click the "My account" link in the top menu) and click on the "Notifications" tab. There you will find several options for customing your notifications. You can customize how frequently you are notified of posts, which method you are notified by (PM or email), and whether or not you want to be automatically subscribed to posts you create or comment on. You can also add or delete subscriptions from there as well.